This invention relates to a method for making an etch-resistant stencil on a metal work piece, particularly a metal sheet. The novel method may be used, for example, in a method for etching an apertured mask for use in a color television picture tube.
The preparation of apertured masks by photoexposure and etching has been described previously; for example, in U.S. Pat. Nos. 2,750,524 to F. G. Braham, 3,199,430 to S. A. Brown, 3,313,225 to N. B. Mears, and 3,751,250 to J. J. Moscony et al. In a typical process, a thin metal sheet, such as of a cold-rolled steel or a copper-nickel alloy, is coated on both major surfaces thereof with a light-sensitive resist or enamel. The coatings are exposed to light images as by contact-printing exposure to render the exposed portions thereof less soluble in a particular solvent. The exposed coatings are developed to remove the more-soluble unexposed portions thereof, and then baked to render the retained, less-soluble, exposed portions etch resistant. Then, the sheet with the etch-resistant stencil thereon is selectively etched as desired, after which the stencils are removed from the sheet.
In prior methods, the light-sensitive resist is usually comprised of fish glue and an alkali-dichromate photosensitizer therefor. The resist is applied from an aqueous coating composition to the metal surface and then dried. In some prior methods, casein has been substituted for fish glue. Where casein has been substituted, the liquid coating composition has a pH of more than 7.0, usually about 9.0 to 10.0, in order to render the casein sufficiently soluble to deposit an adequate amount of material to produce an etch-resistant stencil. Such high pH values reduce the photosensitivity of the resist and therefore require excessively long exposures for producing the stencil.